1. Field of the Invention
The present invention relates to an electrostatic actuator in which a movable piece disposed on a substrate can be driven by an electrostatic force between electrodes.
2. Description of the Related Art
Generally, electrostatic actuators are used in optical communication devices such as optical switches, optical shutters, optical attenuators, angular velocity sensors, resonators, and suitable devices (for example, see U.S. Pat. No. 6,315,462).
Conventional electrostatic actuators include a substrate, a movable piece supported on the substrate, a stationary electrode composed of a plurality of electrode plates disposed on the substrate in a comb-like pattern, and a movable electrode composed of a plurality of electrode plates provided for the movable electrode. The plurality of the electrode plates of the movable electrode extend toward the electrode plates of the stationary electrode to have a comb-like pattern. The movable electrode causes the movable piece to be displaced by an electrostatic force generated between the stationary electrode and the movable electrode.
The movable piece is supported on the substrate by supporting beams in such a manner that it can be displaced in a predetermined direction. The stationary electrode and the movable electrode are formed, for example, by etching a silicon material. A plurality of the formed electrode plates having lengths substantially equal to each other are arranged in comb-like patterns. To drive the movable piece, a voltage is applied between the stationary electrode and the movable electrode. The electrode plates of the movable electrode are attracted the electrode plates of the stationary electrode. The stationary electrodes and the movable electrodes are meshed with each other. Thus, the movable piece is displaced.
While the electrodes plates of the stationary electrode and the movable electrode are meshed with each other, they overlap and are opposed to each other. Thus, electrostatic forces act on the electrode plates not only in the length direction (displacement direction of the movable piece) of the electrode plates but also in the direction (width direction of the electrode plates) that is perpendicular to the displacement direction. Therefore, for example, stationary electrodes and movable electrodes are arranged in a right-left symmetrical pattern with respect to the axial line in the displacement direction of the movable piece. Therefore, electrostatic forces in the width direction applied to the respective electrode plates on the right and left sides cancel each other out.
For example, the electrostatic actuator having the above-described configuration is used for an angular velocity sensor, a resonator, or suitable device. In this case, an AC voltage or the like is applied across a stationary electrode and a movable electrode. Thereby, the movable piece of the actuator can be vibrated in its resonant state of which the displacement (amplitude) is in the range of about several μm to about 20 μm.
In the case of electrostatic actuators used in optical switches or other optical device, a mirror provided on a movable piece is caused to enter or emerge from a light path so that a light beam is reflected or transmitted. Thus, the optical path is switched over. In this case, it is necessary to displace the movable piece (including the mirror) by a distance that is at least larger than the diameter of a light beam so that switching is securely carried out. Thus, in many cases, the displacement is in the range of about 20 μm to 100 μm.
Accordingly, in the conventional actuator the stationary electrode and the movable electrode are formed in a symmetric pattern. When the conventional electrostatic actuator is operated, the movable piece is displaced in a predetermined direction and the electrostatic forces in the width direction (in the direction perpendicular to the displacement direction) cancel each other out.
However, the sizes and shapes of the stationary electrodes and the movable electrodes tend to be dispersed when they are formed, due to errors in size, processing, or the like. Accordingly, in many cases, the shapes of these electrode plates are slightly unsymmetrical.
As a result, when the actuator is operated, the electrostatic forces in the width direction applied to the respective electrode plates of the movable electrodes are not sufficiently cancelled out. In some cases, the movable electrode is displaced so as to be inclined and come into contact with the stationary electrode. Therefore, in the conventional actuator, problems occur in some cases in that the movable piece comes into contact with the stationary piece so that the movable piece can not be sufficiently displaced, or the operation of the actuator becomes unstable. Thus, the reliability of the conventional actuator is reduced.
In particular, for electrostatic actuators such as optical switches or other optical devices, it is necessary to cause a large displacement of the movable piece in its static state. That is, in contrast to conventional actuators used in angular velocity sensors or suitable device, conventional actuators used in optical devices are not used in their resonant state. Thus, when the conventional actuator is operated, the electrode plates of the stationary electrode and of the movable electrode overlap each other over relatively large lengths thereof.
Therefore, large electrostatic forces depending on the overlapping lengths of the electrode plates are applied to the movable electrodes in the width direction. Thus, problematically, because the electrostatic forces can not be cancelled out sufficiently by the electrode shape or by other suitable means, the operation of the conventional actuator becomes insufficient.